Global warming is receiving world wide attention, and means to alleviate its harmful consequences are very much in focus. Energy used for heating, cooling, ventilation and lighting of built environments amounts to a considerable portion of the available primary energy in the world. In particular, the energy demand for cooling by air conditioning has grown very rapidly during the last years. The growth in the energy expenditure is based on increasing demands for indoor comfort. The temperature is requested to be kept within a narrow temperature interval and outdoor contact via large windows and glass façades is considered as an important factor for experiencing comfort.
One way to improve the situation is to have building envelopes with variable throughput of visible and/or near infrared light, e.g. of solar energy. This is often referred to as smart windows solutions. Smart windows can make use of a range of chromogenic technologies, where the term chromogenic is used to indicate that the optical properties can be changed in response to an external stimulus. The main chromogenic technologies are electrochromic (depending on electrical voltage or charge), thermochromic (depending on temperature), photochromic (depending on ultraviolet irradiation), and gasochromic (depending on exposure to reducing or oxidizing gases).
The different techniques are in prior art well known as such and have also been studied in the concept of controlling transfer of energy through different optical devices. One example of the use of chromogenic substances for varying a transmittance can be found in e.g. the U.S. Pat. No. 4,902,108. In the U.S. Pat. No. 5,525,430, the temperature of a thermochromic substance is controlled by resistive heating, making it operating as a kind of electrochromic device. In the patent application US 2005/0002081, an active system controlling transmission can consist of an electrochromic layer or a thermochromic layer. Electrochromic devices are in prior art mainly utilized when visible transmission is in focus. Thermochromic devices are instead primarily directed to approaches where near infrared transmission is of most importance.
In the patent abstract JP05297417, a thermochromic and an electrochromic layer are laminated into an integrated device. A combination of some of the benefits of the different techniques may thus be achieved. However, there are remaining problems with optical devices using laminated thermochromic and electrochromic layers. In certain situations, the impinging light power may heat the laminate to a temperature considerably above the transition temperature of the thermochromic film, thereby causing a screening of the near infrared wavelengths. This may be the case even in cases where the ambient temperature is far below the transition temperature and transmission of the near infrared light would be beneficial for e.g. heating purposes.